Quite a few years ago when I first
began the transition from being a fish-only marine aquarist
to a reef aquarist, I became particularly fascinated with the
tridacnid clams. Aside from their obvious beauty, a large part
of my interest was due to the fact that I found it simply amazing
that such a wide array of color schemes and patterns could be
exhibited by a mere handful of species. While making the clams
aesthetically intriguing, this variety of appearances also led
to the early realization that it was rather pointless to simply
glance at the decorated, fleshy extensions of many of the clams'
bodies in order to attempt to identify these animals to species.
In fact, as I would learn, it is the shell of each specimen
which is typically the key to identification, not just the flesh.
However, with the help of a knowledgeable friend, Nancy Stone,
and a handful of clam shells, it was easy work to learn the
key features to look for, and to make accurate identifications.

Like all other clams, these fascinating
animals are placed in the Phylum Mollusca, along with the
snails and cephalopods, and a number of other related organisms.
Within this phylum, the tridacnids are placed within the Class
Bivalvia (two valves or halves), which also includes the oysters,
scallops, and cockles, etc. The vast majority of these are
filter-feeders, which use specialized dual-purpose gills to
capture tiny food particles from seawater that is circulated
through the interior of their bodies, and to carry out gas
exchange. However, the tridacnids also acquire nutrition through
the harboring of internal algal symbionts. Just as the reef-building
corals do, the tridacnids maintain populations of single-celled
zooxanthellae in parts of their bodies that can produce photosynthesis-derived
"food" for a clam host when provided with sufficient
sunlight.

All clams have a specialized tissue structure
called the mantle which forms a thin, taco-shaped flap that
envelops the whole of the clam's body, and is also responsible
for the precipitation of calcium carbonate to form the clam's
shell. It also typically forms or houses sensory apparatuses
like tentacles and light-sensitive eye-spots, and the openings
through which seawater enters and leaves the body chamber
inside the shell. Yet, unlike almost all other clams, the
tridacnids have greatly oversized mantles, and this is where
the clams' complement of zooxanthellae are maintained. Thus,
this enlarged mantle tissue is typically extended well outside
the edges of the shell to act as something of a solar collector,
increasing the mantle's surface area exposed to sunlight and
therefore enhancing photosynthesis. It is these extensible
mantle flaps which make the clams so attractive, often covered
with exotic patterns of dots, circles, stripes, and waves
in a broad spectrum of colors.

As mentioned, the problem in identification
of clams is that these patterns/colors of the mantles may
vary greatly from one individual to the next, even if they
are the same species and are found in close proximity to one
another. A further complication is the fact that any one species
may have numerous combinations of colors and patterns; some
of those may look very much like one or more of the similar
combinations of a different species. Admittedly, with experience,
some hobbyists can indeed identify some of the more common
tridacnids simply by the appearance of the mantle, but such
identification can actually be quite tenuous at times when
not armed with the knowledge of other diagnostic features.

For these reasons, it is easily understandable
that hobbyists typically have difficulty identifying the tridacnid
clams at the species level. There are relatively few literary
references available to hobbyists and, to make matters worse,
I have personally seen countless wrongly-named specimens for
sale in retail stores. While working in various capacities
for collectors and trans-shippers in the past, it was not
overly uncommon to find clams misidentified from their sources,
as well. However, I should add that I have noticed a substantial
decrease in misidentifications in the last few years, as tridacnids
are being farmed and raised in captivity by knowledgeable
operators.

With these complications in mind, I have
provided as much information as possible about each of the
species common to the hobby to use when a specimen is unidentified,
or when a given identification is in question. The guidelines
below should be of great help and, while many of these can
be variable to some degree, even within a single species,
those features that are the strongest indicators are noted
with an asterisk (*). These noted features are relatively
inflexible within a species and should be given the greatest
weight during the identification process.

Observable features of the shell which may
serve as identification aids:



the nature of the shell's
overall shape and symmetry



the nature of the shell's
ribs or folds, which are the undulations that give the
shell a corrugated appearance



the nature of or lack
of scutes, which are shelf or scale-like structures on
the shell



the nature of the shell's
upper margin which gives the two valves an interlocking
appearance



the nature of, or lack
of, the shell's byssal opening, which is an opening at
the bottom of the shell where numerous tough fibers (collectively
called a byssus) can be secreted to attach the clam firmly
to the substrate

Observable features of the mantle which
may also serve as identification aids:



the degree of, or lack
of, extension of the mantle beyond the top margin of the
shell



the type of, or lack of,
tentacles/projections around the edge of the incurrent
siphon/aperture, which is the centrally-located opening
in the mantle where water is brought into the body cavity.

Figure 1: Shell symmetry is
seen as how similar in dimensions a shell is in both
directions away from its center. For example, symmetry
can be determined by looking at the length of the two
heavy lines drawn from the center of this Tridacna
squamosa shell. Both lines are approximately the
same length, thus this shell is very symmetrical. The
more unequal in length, the less symmetrical a shell
would be. Also labeled are: (A) the scutes, (B) the
ribs, and (C) the upper margin.

Figure 2: The byssal opening (A), when present,
can be seen as an opening on the underside of the shell
and can vary greatly in size. This Tridacna crocea
shell has a very large byssal opening. Also labeled
is the upper margin (B) as seen from the underside of
the shell, through the byssal opening. The margin of
this shell forms a tightly interlocking closure.

Figure 3: The exhalent siphon, through which
water is expelled (A), the incurrent siphon (B), and
the beautifully-colored and extended mantle (C) of this
Tridacna maxima specimen can all be seen clearly.

shell is strongly asymmetrical in
form, typically being much longer than tall*



shell typically has 5 distinct ribs



ribs have numerous very tightly-spaced,
but light scutes; however, these are typically eroded
away by the burrowing activities of this species when
in their natural habitat. Thus, specimens that have been
collected "in the wild", typically have numerous
scutes present only on the upper portion of the shell.
Those raised in captivity are not provided the opportunity
to burrow into substrates and thus retain most, or all
of the scutes.



upper margin is strongly curved and
each valve is symmetrical to the other*



byssal opening is variable in size,
being moderate to relatively large



mantle extension can be well past
the margin, completely hiding the shell and scutes



incurrent siphon is ringed with numerous
small, simple tentacles*

Tridacna maxima is occasionally confused with T.
squamosa. However, the overall elongation/asymmetry of
the shell, the closely spaced nature of the smaller scutes,
and the presence of small, simple siphonal tentacles of T.
maxima help in differentiating the two.

Figure 9: The underside of
a Tridacna maxima specimen with a relatively
large byssal opening.

Tridacna crocea



most commonly available as 2 to 3
inch specimens



maximum shell length is approximately
6 inches



shell is moderately asymmetrical in
form, typically being somewhat longer than tall*



shell typically has 5 to 6 low ribs



ribs have numerous tightly-spaced,
but light scutes; however, these are typically eroded
away by the natural burrowing activities of this species
when in their natural habitat. Those specimens that have
been collected "in the wild" typically have
no scutes present, or have only a few scutes at the upper
margin of the shell. Those raised in captivity are not
provided the opportunity to burrow into substrates and
thus retain most, or all, of the scutes.



upper margin is moderately curved
and each side is symmetrical to the other*



byssal opening is very large in size*



mantle extension can be well past
the margin, completely hiding the shell and scutes



incurrent siphon is ringed with numerous
small, simple tentacles*

Tridacna crocea is often confused
with T. maxima. In this case, the more exaggerated
elongation/asymmetry of the T. maxima shell is again
a strong identifier. Scutes, when present on T. crocea,
tend to be further spaced and smaller, as well. Also, the
byssal opening of T. maxima, while being large at times,
still is not as large as that of the typical T. crocea,
and is more likely to be considerably smaller.

upper margin is very strongly curved,
and each valve is asymmetrical relative to the other,
forming large finger or tooth-like projections which do
not form a tightly closing shell. This characteristic
becomes more prominent with age*



byssal opening is very small in size
to non-existent *



mantle extension is highly variable
and ranges from not extending past the margin at all,
to extending well past the margin, completely hiding the
shell



incurrent siphon lacks tentacles*

Tridacna gigas is often confused
with T. derasa, especially when juveniles. When comparing
larger specimens, the large tooth-like form of the margin
of the T. gigas shell is a strong identifier. With
other specimens the larger, more prominent ribs of the T.
gigas shell, and the lack of siphonal tentacles help in
differentiating the two.

Figure 14: A living Tridacna
gigas specimen, easily differentiated from Tridacna
derasa by a lack of siphonal tentacles.

Hippopus hippopus



most commonly available as 4 to 6
inch specimens



maximum shell length is approximately
14 inches



shell typically has 7 to 8 distinct
ribs, but may have many more less distinct, minor ribs



shell is asymmetrical in form and
quite distinct*



ribs lack scutes*



upper margin is strongly curved and
each valve is symmetrical to the other*



byssal opening is very small in juveniles
and is lost completely in adults



mantle does not extend past the margin*



incurrent siphon lacks tentacles*

Figure 15: A living Hippopus
hippopus specimen, easily identified by its unusual
shell form and recessed mantle, which does not extend
past the shell's upper margin.

A Final Note:

Lastly, I should also mention that there
are a few lesser-known tridacnids, such as T. rosewateri,
T. tevora, and H. porcellanus, but these are
rare and are not often seen in the hobby, thus they are not
discussed here. However, it is also important to note that
there are a number of hybrid tridacnids occasionally offered.
These are individuals that share characteristics of more than
one species, which are the product of the mixing of genetic
material between two species. Tridacnids, like so many other
marine organisms, are broadcast spawners which can eject hundreds
of thousands of freely-mixing sperm and eggs into the water
in the process of sexual reproduction. So, it's easy to imagine
the occasional intermingling of genes if the ability to cross-fertilize
is present. Among these, some of the most prevalent hybrids
seen in the hobby are crosses between T. derasa and
T. gigas. These specimens typically have shells more
like those of T. derasa and have relatively large,
elaborate tentacles surrounding the incurrent siphon like
T. derasa, but have coloration patterns that are clearly
a mix of those commonly exhibited by the two species. Unfortunately,
because the degree of outward, physical expression of the
hybridized genetic material can be highly variable, as you
might guess there are occasional clams which seem to defy
most, or even all identification guidelines. Fortunately,
for those easily frustrated by not having an answer for every
question, these clams are typically few and far between.

If you have any questions about this article, please visit my author
forum on Reef Central.